1. Field of the Invention
The present invention relates to a monolithic varistor, and particularly to a monolithic varistor used for protecting electronic equipment from surge (abnormally high voltage).
2. Description of the Related Art
In order to cope with recent miniaturization of electronic equipment and increased signal-processing speed, electronic parts have been surface-mounted more frequently, and their operation frequencies have been increased. A non-linear resistor serving as a noise absorber is not an exception to this trend; a surface-mount-type varistor formed mainly of zinc oxide (ZnO) or strontium titanate (SrTiO.sub.3) has been put into practical use.
As a measure for reducing the size, especially the height, of a varistor, there has been proposed a method in which a plurality of varistor material layers and a plurality of internal electrodes are layered in order to form a monolithic varistor. However, in the case of a varistor that must have a varistor voltage of 100 V or greater, the distance between adjacent internal electrodes (hereinafter referred to as an "inter-internal electrode distance") in the direction perpendicular to the layered varistor material layers and internal electrodes must be increased, so that employment of a layered structure is difficult.
However, thanks to recent improvements on varistor materials, the varistor voltage per unit inter-internal electrode distance has been increased, making employment of a layered structure possible in terms of varistor voltage. However, there has arisen a new problem that an increased varistor voltage causes a drastic decrease in maximum surge current that can be withstood by the varistor. Thus, the size of layered varistors cannot be decreased, and only varistors having a size similar to that of a single-layer-type varistor can be produced.